Hi Guys just picked up this one for $100 CAD, all the capacitors actually looks good and short of checking for shorts in the capacitors there is not any way that i can test these while still soldered to the PCB. For the record i used some deoxit type spray and some rubbing alcohol and the amp works great. I just want to know everyone's thoughts on replacing all the caps when their are no signs of bulging caps or leaking (this amp is from 1983). Everything appears to sound and work well, however i wouldn't know what it sounded when new. I did just purchase all the electrolytic caps for the amp, just have not yet decided to store them...maybe changing out the big power caps close to the AC side or install of them.
Technics SU-707 Receiver
- Thread starter trivium91
- Start date
Sorry what does this mean? I think at minimum i will replace the big power caps on the AC side, so you guys are saying to just leave all the other caps alone? I can maybe check for shorts since i dont know any other way to check caps that are in the circuit, if there is no short's just leave them?
Oh by direct coupled you are saying none are in the signal path, so nothing will affect the sound if they are out of spec...so just leave them alone until they pop or when the receiver stops working? I think at minimum though would it not be a good idea to replace the big AC power caps as these could be aging and cause the transformer/ power supply to work harder to deal with the spikes of current draw? Just for the record i was not touching any other caps other than the electrolytic capacitors.
MacPhantom
Well-Known Member
Forget blind capacitor replacements, this has just about never an effect. What I would do instead is to align the amp according to the service manual (DC offset, etc.). Waaay more important than replacing components that are working just fine.
I know in car audio when you dont have capacitors in between a sub for example its harder on the alternator and can cause dimming lights. So i just thought that replacing the big power caps would make sense since they probably dont store the amount they did when new causing more strain on the transformer. Would this statement be incorrect? I can check the alignment as you said...although i have never done this im sure the service manual will walk me through it. Why does everyone put such a high priority on replacing all the electrolytic caps than? Or is this not affecting my amp since its direct coupled?
NVM it looks like that was to verify the input voltage, i dont think i need that. So basically i need to plug in my multimeter and adjust the appropriate dials for left and right channels (with a multi meter installed in speaker ports) so that it says 2MV on both channels just after warm up (5-15) seconds. After that it says the microprocessor will control the voltage. So i guess this is what you mean by DC offset voltage. I cant check the protection circuit functions though as it says i need to generate a 1 KHz tone at 1.5V...i dont have anything that is capable of doing that. Some sort of tone generator?
MacPhantom
Well-Known Member
Sure, this is true; power supply capacitors act as some sort of filter and reservoir. Your Technics hence has two big filter caps; since they're picked with quite a safety margin in terms of max. voltage, chances are extremely low that they are bad, especially as they're not even bulging or leaking, and since the sound quality appears to be just right.
The main effect of bad power supply caps is an audible hum in the signal path (120 Hz in the US and parts of e.g. Japan, 100 Hz elsewhere). Also, one *might* hear a lack of bass at higher volumes. However, the problem has to be very severe to cause such effects; if you cannot hear anything wrong, the caps are fine.
I can tell you: people get to hear that capacitors can go bad, especially under high temperature. Then they go to their local electronics store and find that caps are very cheap components, easy to replace (no special tools or alignment needed). Finally, they like to tinker and fall for autosuggestion; i.e., they automatically expect a sound quality improvement ("new components must be better, right? It's like in cars blabla"). As a result, they greatly exaggerate the minor probability of a cap going wrong and just stupidly replace every capacitor they can find.
Statistically, I am pretty sure that 99.5% of all caps that have ever been replaced in a blind swap marathon were still perfectly fine. In fact, if you examine caps more closely, you will find that older models were manufactured at a higher quality standard than modern ones. Very often, you'll find that e.g. a 470uF capacitor from 1980 still holds this value or even more, whereas new ones from today have only 90%, sometimes even less of the indicated capacity.
Don't get me wrong - replacing caps can be unavoidable, and as I said, caps do go wrong from time to time. However, in most of the cases, this is either because of one of three problems:
- the caps were placed right besides hot components like voltage regulators and exposed to high temperatures over many years. In this case, it makes sense to replace them with 105°C types.
- the unit was designed in a bad way and caps were installed with not enough voltage tolerances. Good example: Onkyo M-5590 power amp: the eight power supply caps are rated for 90V; the power supply rail runs at 88V. Two volts tolerance is just not enough for such huge cans!
- the caps are known to turn bad at some point. Examples: some tantalum ones, caps made by the Roederstein company, and finally, those made by Rifa. Yamaha for instance used the latter in their integrated amps, and since they do go wrong, the company offers a service free of charge to replace them for their amps from the 80s - to this date!
Edit: some words on direct coupling: especially earlier amps were not direct coupled; instead, they had a capacitor in series in between the output of the power transistors and the speakers. The reason for this cap was to block DC voltage coming from the output stage (you don't want DC to go into your speakers, or they're going to be burnt in a few seconds).
At some point in time, engineers got rid of coupling caps (caps are said to cause phase shifts and people thus don't like to have them in the signal path). For this, they had to eliminate the DC coming from the output stage. They usually do that by feeding the output stage with two voltages, one positive and one negative, and "sandwiching" the output stage right in between. Like that, you don't need an output cap anymore (as in your SU-V707), but at the same time you'd have to be very careful to align the amp properly and to choose matching pairs of transistors. Any misalignment can cause some residual DC to be output into the speakers; hence these amps urgently require a good DC speaker protection.
As I said, your amp is DC coupled. Amps since the mid 70s are usually not really problematic and quite stable (with some exceptions). However, it still can't hurt to align the unit properly. So make sure to follow the instructions carefully in the service manual (don't slip up with the voltmeter probes).
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Wow thats an awesome reply! I believe a capacitor somewhere is bad as when i tried to adjust the bias voltage for left and right channels on the IC, the meter would jump around...when i turned the dials it would jump around and go back to zero...so im not sure what that means. I decided to plug into the back of the speaker outputs and im getting anywhere from 2-10mv when i adjust the left and right channels so its a mixed bag really...doesnt seem consitant with anything...would this not indicate a faulty capacitor somewhere?
Yeah the service manual said to adjust to 2mv, but everytime i try to adjust it the level jumps around and goes back to zero.
Lol you sound like Yoda. Im just trying to set the Bias voltage (ICQ) than after that i can deal with testing the protection circuit and what not. The main issue im having with the bias voltage is it keeps returning to zero, maybe im supposed to wait 10 minutes with the amp on first but the manual says only 5-15 seconds unless im reading it wrong. It a bit hard to understand since i think it was translated from Japanese originally.
Maybe someone can help me get on the right track? the service manual is here:
https://www.dropbox.com/s/i6gvq0omoyw8va1/technics_su-v707.pdf?dl=0
Its also very possible that my multi meter is out of calibration or is having trouble accurately reading voltage as low as 2MV
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Sorry im new to amps and what not, should i not be following the service manual? Or are they describing something different...as i believe im following the service manual as it states for the Bias (ICQ) Voltage. You are referring to the two white ceramic pieces with three pins on each soldered to the pcb? Thats the points the service manual says to measure each for right and left channels.
Just googled emitter resistor it looks like i am measuring across the emitter resistor which is also where the service manual is showing to measure, there are two white ceramic things that look like this:
So is it possible i have a bad capacitor somewhere? Or is there an issue with my multi meter registering low 2mv increments? Or should i be waiting 10 minutes to take the measurement even though the manual says 5-15 seconds.
Definately DC, i also borrowed a fluke scopemeter 123 from work...its basically a portable oscilloscope. I have no idea how to use it but hopefully it can detect very low voltages and maybe i can use it to see if there is any oscilation in the amp indicating problems. If it does not detect the voltage than i guess ill wait for the multimeter i bought to come in. Hopefully i can get something going here... what mutli meter do you use to set your 505? Also what voltage do you set it at?
20mv? Maybe my service manual has a typo...maybe its supposed to be 20 mv and not 2 mv? I will try and clean the pins and report back. Its an autoranging meter so there is no selections.
I just checked your service manual for 505 and it says to set to 2mv not 20mv?
